The invention relates to synchronous rectifiers, and more particularly to improved low resistance electrical interconnections for such rectifiers.
Synchronous rectifiers employing power field effect transistors as the rectifying elements are commonly used in applications such as power supplies, wherein substantial levels of current are provided. An example of one such power supply is described in the paper "The Design of a High Efficiency, Low Voltage Power Supply Using MOSFET Synchronous Rectification and Current Mode Control," by Richard Blanchard and Phillip E. Thibodeau, IEEE Power Electronics Specialist Conference, pages 355-361 (1985).
As semiconductor manufacturers improve (decrease) the "resistance-drain-to-source on" (RDS ON) of power FETs, the method of interconnection of the devices in the hybrid circuits becomes of critical importance. With the present state of the art, some power FETs have RDS ON values which are as low as 10 milliohms. Placing four devices in parallel reduces the effective RDS ON resistance of the circuit to 2.5 milliohms. However, at this level of resistance, the interconnection of the FET device to the package by conventional wirebonding techniques and the package to external circuits by conventional lead frame or pins introduces approximately 10 to 15 milliohms of resistance. This relatively high interconnect resistance introduces unwanted voltage drops, which produce additional heat that must be removed to maintain reliability. Further, the efficiency of the rectifier is reduced by such unwanted interconnect resistance. For example, assuming a total circuit resistance (resistance due to the circuit package, components and interconnects) of 12.5 milliohms, and a current level of 100 amperes, the circuit resistance leads to a voltage drop of 0.125 volts. Power lost in synchronous rectification is 12.5 watts.
There is accordingly a need for a means of interconnecting the FETs in a synchronous rectifier to reduce the packaging resistance to improve the efficiency.
It would further be desirable to provide a means of device interconnection in synchronous rectifiers which provides very low interconnect resistance.